Coronary artery disease is one of the leading causes of death worldwide. The ability to better diagnose, monitor, and treat coronary artery diseases can be of life saving importance. The decision to place a stent in a coronary artery depends on the amount of occlusion of the artery by a plaque, which is most often assessed by angiography according to quantitative measures of vessel stenosis, such as the minimum lumen area (MLA) and percent area stenosis (% AS). The relationship of these geometric measurements to ability of the artery to supply an adequate flow of blood to the myocardium when metabolic demands are high has been a long-standing area of investigation.
The relevance of the minimum lumen area (MLA) as a measure of lesion severity has been debated by experts and prior attempts to use it have not been compelling. For example, attempts to relate MLA and Fractional Flow Reserve (FFR), a standard measure of the physiological significance of a lesion, by simple linear regression and curve fitting of experimental data have achieved little success. Others are developing sophisticated 3D flow models, to predict FFR from magnetic resonance (MR) angiograms. Several significant factors appear to limit standard angiography from predicting FFR from MLA measurements.
First, the accuracy and reproducibility with which cross-sectional areas can be measured with angiography, which generally has a spatial resolution of 0.2-0.4 mm, are relatively low. The angle of the X-ray projection, in addition to the shadowing effect of lesions with irregular contours, can increase errors significantly beyond the theoretical minimums.
Second, when assessing the physiological significance of a lesion and the potential value of revascularization, it is important to account for the normal dimensions of the vessel as well as the minimum cross-sectional area at the site of the lesion. These variables influence the blood flow through the lesion and, hence, the magnitude of the pressure drop caused by a given MLA value.
Third, the hemodynamic effects of a lesion depend on local variations of its cross-sectional area integrated over the entire length of a lesion. Therefore, the minimum cross sectional area alone is insufficient to characterize the pressure drop across a lesion at a given flow rate, especially in patients with diffuse coronary disease.
Fourth, the flow resistance or pressure drop caused by an incremental segment of a lesion depends on its shape as well as its cross-sectional area and length. Especially at high blood flow rates, the eccentricity and local slope of the walls of the artery can influence the effective resistance of a lesion, because losses due to flow separation and turbulence depend on local flow velocity.
Finally, in certain patients, the flow reserve of the myocardium supplied by the vessel can be low, due to microvascular disease, flow through collateral branches, or capillary shunts within infarcted myocardium. Therefore, even if the vascular resistance of a lesion in the vessel is high, revascularization may be contraindicated, because the pressure drop across the lesion may be clinically insignificant.
Optical coherence tomography (OCT) imaging, applied in combination with new clinical parameters based on advanced analysis of lesion morphology, has the potential to overcome many of the limitations of conventional measures of lesion severity based on angiography. Intravascular optical coherence tomography (OCT) is a catheter-based imaging modality that employs safe, non-ionizing near-infrared light to peer into coronary artery walls and present images valuable for the study of the vascular wall architecture. Utilizing broad-band coherent light, interferometry, and micro-optics, OCT can provide video-rate in-vivo tomography within a diseased vessel with resolution down to the micrometer level. This level of detail enables OCT to diagnose as well as monitor the progression of coronary artery disease.
The high resolution of OCT enables accurate measurement of the shape and dimensions of the vessel lumen over the length of the lesion and its adjacent reference segments. Furthermore, advanced models of flow dynamics enable the physiological significance of lesions to be estimated under both normal and hyperemic conditions. The accuracy of OCT even exceeds that of state-of-the-art IVUS imaging systems, which have resolutions of approximately 0.15 mm in the axial dimension and 0.3 mm in the angular dimension. Because static blood obscures the boundaries of tight lesions, IVUS is limited in its ability to accurately measure MLA values below about 1-2 mm2. Given all of the problems relating to existing attempts to measure FFR, a need therefore exists for accurate methods of determining or measuring FFR and other cardiovascular system related processes, and devices.
The present invention addresses this need and others.